Anodized specimens of aluminum and tantalum were deformed in laboratory air; strain to failure and the failure characteristics of the oxide film were evaluated optically. Barrier-type anodie aluminum oxide films of thickness greater than N400A failed at ~0.925% strain normal to the tensile axis and apparently suppressed substrate slip emergence. Thinner anodic films on aluminum failed along substrate slip traces at ~1.12% strain. These films did not suppress slip emergence, but were apparently stronger. The presence of a porous oxide superimposed on thin barrier-type films caused them to fail in the thick film mode; this was the only effect of a porous layer. Anodic films on mechanically polished tantalum failed at N0.28% strain, independent of thickness, but showed a failure mode dependence on thickness analogous to that of aluminum. Films on chemically polished tantalum substrates always failed in simple tension, but showed a thickness dependence, failing at ~0.14% strain for thicknesses greater than ~680A, and ~0.20% strain for thicknesses less than that value. Failure of these films was accompanied by separation of the films from the substrate.Environment-sensitive failure processes such as stress corrosion crackir~g and corrosion fatigue involve a complex interplay of chemical and mechanical aggression and are far from well understood. Oxide film failure is seen by many investigators as a necessary prelude to the initiation and propagation of cracks in these processes (1-7). The literature dealing with the mechanical behavior of oxide films is not voluminous, and reported values of mechanical properties, e.g., Young's modulus, film strength, and film failure strain, exhibit large variations. Edeleanu and Law (8) strained thick porous anodic films on aluminum in tension and found them to be elastic, with failure occurring at 1-2% strain normal to the tensile axis. Bradhurst and Leach (9, 10) deformed aluminum specimens bearing barrier-type anodic films, detecting film failure by a variety of electrical methods to occur at 1-3% strain. For 1500A films removed from the substrate, a failure strain of 2.48% and a Young's modulus of ~ 41 • 1,08 MN/m 2 were determined. Also using electrical detection methods, Bubar and Vermilyea (11, 12) found thin anodic films on aluminum to exhibit some ductility, while thick films were brittle. They also observed a large ductility, as much as 50% elongation prior to fracture, in anodic tantalum oxide films tested in this wet manner, in marked contrast to the value of 0.83% reported by Eliezer and Brandon (13), who used a (dry) bulge test technique. In earlier work Young (14) had attributed this discrepancy to the presence of a surface film on chemically polished specimens. This film gave rise to poor adhesion of the anodic oxide, with consequent oxide detachment and brittle behavior, whereas removal of this "preanodic" film by leaching in boiling water resulted in a very adherent, ductile anodic oxide. Grosskreutz (15,16) found anodic aluminum oxide films, both adherent to and s...
